A voltage source converter technique is a novel converter technique based on a turn-off device IGBT (Insulated Gate Bipolar Transistor) technique and a pulse width modulation (PWM) technique. In a low voltage application, a two-level converter composed of IGBTs is widely applied. To address a high-frequency transition of an output voltage of the converter, problems such as high dv/dt stress, large system losses, high noise and harsh electromagnetic environment, may be caused. Therefore, a three-level voltage source converter is proposed, which expands the application range of a voltage source converter in terms of voltage level and capacity.
With the emergence of a modular multilevel converter (MMC), a multilevel converter is successfully applied in the field of Voltage Source Converter-High Voltage Direct Current Transmission (VSC-HVDC), where the voltage level reaches hundreds of kilovolts, and the capacity reaches several gigabytes.
However, all voltage source converters have an inherent defect in which a direct current (DC) fault cannot be effectively handled. When a short-circuit fault occurs at a DC side, even if the converter has been blocked, an (alternating current) AC power supply still can inject a current to a short-circuit point via an anti-parallel diode of an IGBT so as to form a freewheel passage, so that fault clearing becomes difficult. Thus, how to solve or relieve the problem of the DC fault of a voltage source converter becomes a key technical factor for solving the problems in the development of DC interconnection technology.
In view of this, the present inventors have proposed a suppression means from a standpoint of the mechanism of DC fault currents of a voltage source converter, resulting in the present application.